Microphone not working

I am not able to get my microphone to work. I have tried all the trouble shooting options (been at it for at least 3 hours (!)).
I use Microsoft 365. When I go to Microsoft Privacy & Security and scroll down to Microphone, the Zoom app does not appear in the list, so there is no switch I can turn on or off. The Zoom app does appear at the botom of the list, but not with a switch, only with a notification saying 'This app has had access to microphone in the past'.
I have tried numerous options, both in Microsoft as in Zoom, but nothing has worked. I can't start work (online teaching) without this, so please HELP!